Canola/rapeseed (Brassica napus L.) is a major oilseed crop in Canada, Europe, Australia, China and the Indian subcontinent. The quality of canola/rapeseed oil is determined primarily by its constituent fatty acids. The major fatty-acid constituents of Brassica oil are palmitic acid (C16:0), stearic acid (C18:0), Oleic (C18:1), linoleic acid (C18:2), linolenic acid (C18:3), arachidic acid (C20:0), eicosenoic acid (C20:1), erucic acid (C22:1). Erucic acid is one of the main fatty acids in rapeseed oil. Low erucic acid in rapeseed improves the quality of the oil because high erucic acid is relatively low in digestibility and has been associated with health problems (Beare et al. 1963). On the other hand, high erucic acid rapeseed (HEAR) has several potential applications in the oleo-chemical industry for the production of high temperature lubricants, nylon, plastics, slip and coating agents, soaps, painting inks, surfactants (Topfer et al. 1995).
In B. napus, additive alleles at two gene loci control the erucic acid content in seeds (Harvey and Downey 1964), and these two genes (E1 and E2) are mapped in rapeseed (Ecke et al. 1995; Jourdren et al. 1996; Thormann et al. 1996). Development of low or high erucic acid content B. napus cultivars requires a long process of crossing, backcrossing and selfing of the segregating generations followed by identification of low or high erucic acid content lines from fatty acid profiles of seed lots from individual seeds by gas chromatography (GC). However, DNA molecular markers which are tightly linked to the erucic acid genes or inside the candidate genes can be applied with high efficiency in marker assisted selection (MAS) for rapid transfer of a character into an otherwise desirable genotype efficiently and effectively. For instance, 15 of 16 plants in the F2 generation of canola/rapeseed crosses could be discarded at the seedling stage by selecting homozygous Bn-FAE1.1 and Bn-FAE1.2 genotypes with high or low erucic acid content instead of growing plants to full maturity and then doing seed quality analysis and selection.
The pathway for erucic acid biosynthesis and the major reactions involved in this pathway has been well characterized in Arabidopsis. Oleic acid is the main precursor for erucic acid biosynthesis via an elongation process in the developing embryos of B. napus (Xiaoming et al, 1998). In seeds of Arabidopsis thaliana it was reported that fatty acid elongase 1 (FAE1) is the candidate gene and that the gene product was essential for elongation of C18:1 (oleic acid) to C22:1 (erucic acid) (Lemieux et al. 1990; Kunst et al. 1992). The elongation process has four different steps. The first step is the condensation of oleoyl-CoA to malonyl-CoA to form a 3-ketoacyl-CoA. The second step is the reduction of the 3-ketoacyl-CoA to produce 3-hydroxyacyl-CoA. The third step is the dehydration of the 3-hydroxyacyl-CoA to form trans-(2,3)-enoyl-CoA. The final step it is the further reduction of the trans-(2,3)-enoyl-CoA. These reactions are catalyzed by four different enzymes 3-ketoacyl-CoA synthase, 3-ketoacyl-CoA reductase, 3-hydroxyacyl-CoA dehydratase and trans-(2,3)-enoyl-CoA reductase, respectively (Fehling and Mukherjee 1991). The role of FAE1 gene in producing erucic acid was genetically ascertained by genetic transformation of a low erucic acid content rapeseed (Lassner et al. 1996). In rapeseed, the two loci E1 and E2 of FAE1 homologs encode the rapeseed 3-ketoacyl-CoA synthases for the elongation process to generate erucic acid from oleoyl-CoA (Barret et al. 1998 and Fourmann et al. 1998). In B. napus, these two homologs of the FAE1 gene (Bn-FAE1.1 and Bn-FAE1.2) have been characterized. These two homologs show 99.4% nucleotide identity and a two-base deletion in the low erucic acid content line results in a functional loss of Bn-FAE1.2 gene in the C genome (Fourmann et al. 1998). Katavic et al. (2002) reported that single amino acid serine at 282 positions in high erucic acid content line is substituted by phenyl-alanine in low erucic acid content line due to one base change in the Bn-FAE1.1 gene in the A genome. In this report, BAC clones containing Bn-FAE1.1 and Bn-FAE1.2 genes from the A and C genome libraries were used to extend the sequence on the outside of these two genes to develop genome specific high throughput molecular markers. These markers will considerably facilitate the selection of the four different erucic acid content control alleles in canola/rapeseed breeding programs.